It is nearly impossible to keep water free from all traces of contaminants. According to the Environmental Protection Agency, even drinking water “may reasonably be expected to contain at least small amounts of some contaminants”.
Those “small amounts of contaminants” are no issue for the most of us. Adults in particular are unlikely to be adversely affected by them.
Children are not quite as impervious to contaminants as adults however. As noted by the World Health Organization, children are more vulnerable to environmental risks because they are still developing.